This invention was made at the Lawrence Berkeley Laboratory under Contract No. W-7405-ENG-48 between the Energy Research and Development Administration and the University of California.
This invention relates to the preparation of thick film conductors, particularly noble metal thick film conductors.
Hybrid integrated circuits are becoming increasingly important as a result of the miniaturization of electronic components, circuits, and systems. Thick films are extensively used as capacitors, conductors, and resistors in these hybrid circuits. They are also used in high power television circuits, microwave devices and other integrated circuit applications. Thick films have been used not only because they are reliable but also because they are economical.
A conductor thick film is essentially a film of a composite of glass and a metal, usually a noble metal, on a ceramic substrate, having a thickness of the order of a few mils. The glass content is small (about 15 w/o) in conventional films, and its purpose is to provide good bonding between metal and substrate. Thick films are differentiated from thin films both in mode of preparation and in their thickness. Thin films are essentially of a thickness ranging from a few thousand A to a few microns and are normally vacuum evaporated, sputtered or chemically deposited.
Conventionally, conducting thick films having a mixture of glass and metal particles in a suitable proportion in an organic vehicle with a suitable viscosity are printed onto a substrate. The glass, usually a low softening glass, with a composition having an expansion coefficient less than that of the substrate (for example, 96 w/o Al.sub.2 O.sub.3) is powdered to a micron size (1 .mu.m-10 .mu.m range) and mixed with the micron size (1 .mu.m-10 .mu.m range) metallic powder, normally a noble metal. Usually the metal content varies from 70 to 90 w/o of the solids. This mixture is then suspended in an organic vehicle which consists of a resin binder, solvent and additives to give a suitable viscosity. The liquid ratio is normally about 75 w/o depending on the viscosity. The liquid-solid mixture is screen printed onto an alumina substrate, and the system is dried and fired for a specific time at a given temperature to give a conducting thick film.
Conventionally, the purpose of the glass is to provide good bonding between the substrate and the metallic film and to hold the metallic particles together in the film. The microstructure of the fired system has glass particles interspersed with metallic particles in contact with one another as shown in FIG. 1a.
Because the present commercial thick film conductor compositions use about 70-90 w/o metal in the solids composition thick film circuits are a significant user of precious metals. Thus, a method which makes possible a substantial reduction in the amount of precious metal needed for good conductivity would be desirable.